What Can a Backup Battery Power? Choosing Emergency Loads
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Author
Andrew Blok
Electrification and Solar Writer and Editor
When a storm knocks out the grid, the last thing you want is to discover your battery ran out of power two hours in. The good news: with the right setup, a home battery can keep your most important appliances running much longer. The key is choosing which loads to back up.
"Load" is the industry term for any use of electricity — your refrigerator, your WiFi router, your security camera. Most homeowners don't back up their entire home. They back up the things that matter most. Here's how to think about it.
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What should you back up?
Not all appliances are equal when it comes to battery backup. Some draw so much power they can drain a battery in hours; others sip electricity and barely make a dent. Here's a practical way to think about your options:
Tier 1 — Almost always backed up Refrigerator, lighting, phone chargers, WiFi router, medical devices
Tier 2 — Common choices Microwave, TV, laptops, security cameras
Tier 3 — Think carefully Heat pumps, window AC units, well pumps
Tier 4 — Rarely backed up EV chargers, electric dryers, electric water heaters, central AC
Heating and cooling equipment sits in Tier 3 for a reason: it has an outsized impact on how long your battery lasts. More on that below.
Why not back up everything?
There are three reasons most homeowners focus on essential loads rather than whole-home backup.
Cost: Sizing a system to run your entire home normally can dramatically increase installation costs since you’ll likely need more batteries than otherwise. Focusing on essential loads gets you the most resilience for your dollar.
Capacity: Batteries store a finite amount of energy, measured in kilowatt-hours (kWh). High-draw appliances can deplete that supply quickly.
Surge power: Some appliances — especially motors like those in heat pumps — require a large burst of power to start up. Your battery needs to handle that instantaneous peak, not just the average draw.
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How long will a battery actually last?
The honest answer: it depends on what you're running and whether your solar panels are generating power. Here's how one planning software estimates that different choices will play out in practice.
| Setup | Sunny weather | Cloudy weather |
|---|---|---|
| Small battery (5 kWh) + heat pump | 9+ hours | 7 hours |
| Small battery (5 kWh), no heat pump | 7+ days | 18 hours |
| Large battery (10+ kWh), no heat pump | 7+ days | 49 hours |
| Large battery (10+ kWh) + heat pump | 7+ days | 19 hours |
Estimated runtimes based on backing up a refrigerator, WiFi, phone chargers, television, microwave, lighting, and two laptops.
The heat pump's impact is striking. Removing it from a small battery system extends cloudy-weather backup from 7 hours to 18 — more than doubling your runway. A larger battery can handle a heat pump more easily, which is why homeowners who want heating and cooling backup typically size up.
For another reference point: Tesla estimates that a Tesla Powerwall 3 (13.5 kWh) paired with 5.7 kW of solar provides an estimated 7+ days of backup in sunny weather and up to 18 hours during an average winter.
Is a battery right for you?
A backup battery makes the most sense if any of these apply:
- You live somewhere prone to outages. Wildfires, hurricanes, and aging grid infrastructure all make resiliency more valuable.
- Your utility charges time-of-use rates. A battery lets you power your home from stored energy when grid electricity is most expensive.
- You have solar and want to use all of it. Without a battery, excess solar generation goes back to the grid. With one, you use it yourself.
The best way to understand what a battery can do for your specific home is to talk to an expert. Palmetto advisors can help you figure out how much capacity you need — or whether a battery is the right fit at all. Reach out for a free quote today.
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Frequently asked questions
Can I add more backup loads later?
It's usually easier to decide on your critical loads during initial installation. Adding circuits later can require additional electrical work and added cost.
Will my solar panels work during a blackout if I don't have a battery?
No. Standard solar systems automatically shut down during outages to prevent electricity from back-feeding into the grid while utility workers make repairs. You need a battery and a transfer switch to keep your home powered independently.
What is a home battery's lifespan?
Most modern lithium-ion home batteries come with a 10-year warranty and are designed to retain about 70% of their original capacity by the end of that period.
Disclaimer: This content is for educational purposes only. Palmetto does not provide tax, legal, or accounting advice. Please consult your own tax, legal, and accounting advisors.
